In the art of recovery of hydrocarbons such as oil and natural gas from subterranean formations through a wellbore penetrating the earth to the hydrocarbon-bearing formation, it is common to perform some type of stimulation procedure to the formation in order to enhance the recovery of the valuable hydrocarbons.
In order to recover the valuable fluids from a subterranean formation, a well is drilled from the surface, or in the case of a subsea well, from the sea floor, to the hydrocarbon-bearing formation. Following drilling, the well is generally completed by installing a tubular well casing in the open borehole and cementing the casing in place by forcing cement down through the casing bore to the bottom of the borehole and then upwardly in the annular space between the casing and the borehole. The casing acts to retain an open conduit between the formation and the surface against wellbore sloughing or formation particle transport which would tend to block the borehole. The cement acts to hold the casing in place in the borehole and also to provide a seal which acts to prevent migration of fluids between different subterranean zones penetrated by the borehole.
Because the casing and cement forms a continuous hollow column, no wellbore fluids are able to enter the well to be transported to and recovered at the surface. For this reason, it is common to provide openings through casing and cement annulus in the zone of interest by perforating using precisely placed, high explosive charges which, literally, blast a hole through the casing and cement and out into the surrounding formation to provide a channel from the formation into the wellbore for recovery of the desired formation fluids.
It is also common to stimulate the flow of valuable fluids to the wellbore and the perforated channels by treatment of the surrounding formation. One common stimulation treatment is hydraulic fracturing of the adjacent formation rock. In hydraulic fracturing, an hydraulic fluid typically comprising an aqueous or hydrocarbon base liquid or foam is thickened with a polymeric material and pumped through the casing and the perforations at a sufficiently high pressure to form a crack or fracture in the surrounding formation rock. High pressure pumping is then continued to extend the fracture generally radially outwardly from the wellbore. In order that the fractures which are created do not close once fracturing pressure is released, the fracturing fluid typically contains a solid particulate material, termed a proppant, which will remain in the fracture once the pressure is released to provide a low-restriction flow channel through the formation back to the wellbore.
Typical proppant materials used in hydraulic fracturing comprise sand or a synthetic ceramic-type material having a density of 2.5 g/ml or greater. Because of this relatively high density, the fracturing fluid employed in the proppant placement process must have sufficient viscosity to retain these dense proppant particles in suspension for a sufficient period of time to allow mixing and transport through surface pumping equipment and piping, through the length of the wellbore and outwardly through the full length of the formation fracture. If the proppant settles prematurely in the carrier fluid, the proppant will form a block to extension of the fracture and the further placement of proppant in the fracture. This early termination of the fracturing process may cause the overall treatment to fail short of its goal to provide a conductive channel for the recovery of formation fluids.
In an overbalanced perforating and fracturing treatment operation, the most efficient use of the explosive energy generated in the perforating operation requires that the high-pressure fluid in the wellbore which is released to fracture the formation, flow easily and quickly through the perforations to effect fracture initiation and extension away from the wellbore. Thus, the most efficient fluids for an overbalanced perforation and fracturing operation have both high shear thinning and low viscosity properties since such fluids will flow more easily and quickly. However, the characteristics of shear thinning and low viscosity are incompatible with the suspension, transport and deposition of traditional proppant materials in an overbalanced perforating and fracturing operation. Traditional sand and sintered-type proppant materials prematurely settle out of the low viscosity, shear thinning fluids which are most efficient for rapid fluid movement in a simultaneous perforating and fracturing treatment. For this reason, most current simultaneous perforating and fracturing operations are carried out without proppant being in the wellbore. This results in a less efficient treatment.